The present invention relates generally to array antennas and fabrication methods therefor, and more particularly, to low cost methods of fabricating a true-time-delay continuous transverse stub array antenna.
Previous true-time-delay, continuous transverse stub array antennas were made either by machining or molding microwave circuit features out of low-loss plastics, such as Rexolite(copyright) or polypropylene. The plastic was then metallized to form a dielectric-filled, overmoded waveguide or parallel-plate waveguide structure. Such antennas are disclosed in U.S. Pat. No. 5,266,961 entitled xe2x80x9cContinuous Transverse Element Devices and Methods of Making Samexe2x80x9d, U.S. patent application Ser. No. 08/885,583, filed Jun. 30, 1997, entitled xe2x80x9cPlanar Antenna Radiating Structure Exhibiting Quasi-Scan/Frequency Independent Driving-Point Impedancexe2x80x9d, and U.S. patent application Ser. No. 08/884,837, filed Jun. 30, 1997, entitled xe2x80x9cCompact, Ultrawideband, Matched E-Plane Power Dividerxe2x80x9d.
Subsequently to the above inventions, U.S. patent application Ser. No. 08/884,837, filed Jun. 30, 1997, entitled xe2x80x9cMethods of Fabricating True-Time-Delay Continuous Transverse Stub Array Antennasxe2x80x9d describes an air-dielectric design fabricated from metal or metallized plastic sheets into which the desired microwave circuit features have been formed. The layers are then assembled and joined together using one of several available processes, such as inert gas furnace brazing or ultrasonic welding. However, a flawless bonding process is necessary to assure closure of the seams, as internal inspection and repair are usually not practical once the unit is assembled.
Air-dielectric has several significant advantages over solid-dielectric microwave structures, including lower losses and reduced susceptibility to nonuniformities in the microwave properties of the dielectric, such as inhomogeneity and anisotropy. RF energy does not propagate through the dielectric material. Thus, continuous transverse stub arrays may be fabricated using low-cost materials with excellent physical properties but poor microwave characteristics, such as acrylonitrile-butadiene-styrene (ABS), with metallic surfaces to mimic its conductive surfaces.
A prototype antenna was developed by the assignee of the present using the solid-dielectric approach. The prototype design operates satisfactorily over an extended band of 3.5 to 20.0 GHz. Dielectric parts of uniform cross section were made from Rexolite(copyright) 1422 using conventional machining techniques. The parts were bonded together with adhesive and then all outside surfaces except a line-feed input and the radiating aperture were metallized with a highly conductive silver paint.
The primary disadvantage of the solid-dielectric approach is the dielectric loss, which becomes increasingly significant at higher millimeter wave frequencies. Other disadvantages include variations in dielectric properties, such as inhomogeneity and anisotropy, the high cost of premium microwave dielectric materials, and to a lesser extent, the cost of fabrication, bonding and metallization of the dielectric parts. Air-filled designs also have problems, and in particular, microwave circuit features are internal to the waveguide structure and may be inaccessible for mechanical inspection after assembly. Thus the processes used to fabricate such antennas must insure accurate registration of parts, maintain close tolerances and provide continuous conducting surfaces across all seams.
Accordingly, it would be an advantage to have low cost methods of fabricating true-time-delay continuous transverse stub array antennas that improve upon previous methods.
The present invention provides for improved methods of fabricating air-filled, true-time-delay, continuous transverse stub array antennas comprising extruded sections to form desired microwave circuit features. End plates support the extrusions. The method of the present invention results in highly producible designs that can be manufactured in large quantities at very low cost.
An exemplary method comprise the following steps. A plurality of extruded sections that are physically independent of one another are fabricated. The plurality of extruded sections are arranged in a predefined pattern defining an array antenna structure, wherein adjacent surfaces form waveguides of the array antenna. The plurality of extruded sections are joined together at their respective ends to form the array antenna.
To join the extruded sections together, a plurality of end plates are typically fabricated and then the extruded sections are secured and specially located by the end plates. The plurality of extruded sections and end plates may comprise metal or plastic. If the extruded sections and end plates are plastic, they are metallized using a process such as vacuum deposition, electroless plating, or lamination during the extrusion process. The (metallized or metal) end plates are interconnected to the (metallized or metal) extruded sections to form the array antenna structure.
The present method may use either metal or plastic extrusions to form air-filled dielectric, parallel-plate waveguide structures. To obtain RF conductivity, plastic surfaces are metallized, using processes such as vacuum deposition, electroless plating, or by lamination during the extrusion process. The extrusions may be drawn as thin-walled tubes to minimize weight.
A major advantage of the present invention is that the parallel-plate waveguides formed by the extrusions are completely without seams. This is a major improvement over the layered construction previously cited in the Background section, where parting lines exist between adjacent layers.
The method of forming microwave structures from extruded sections may be generally employed to fabricate ultrawideband antenna feed and aperture architectures used in true-time-delay, continuous transverse stub array antennas. The fabrication processes are mature, and therefore yield designs that can be mass-produced at low-to-moderate cost. Such affordable, wideband antennas are of major importance to multifunctional military systems or high-production commercial products where a single wideband aperture can replace several narrowband antennas.